Polyimides are not only favorable in mechanical properties but also satisfactory in other properties such as heat resistance, resistance to chemicals, and electric insulating properties and have, therefore, been used broadly as electrical/electronic materials, car component materials, and substitutes for metals and ceramics, among other applications.
The conventional process for synthesizing a polyimide comprises reacting a tetracarboxylic dianhydride with a diamine in a solvent such as N,N-dimethylformamide (DMF) to give a polyamic acid which is a precursor of the objective polyimide in the form of a varnish, and subjecting this varnish to precipitation to provide the objective polyimide as fine particles.
However, this technology has the drawback that the polyimide particles separating out with the progress of polymerization reaction undergo coalescence/coagulation, thus failing to give a monodispersed polyimide system.
An alternative technology comprises polymerizing a tetracarboxylic dianhydride with an organic diamine in an organic solvent under heating to give a polyamic acid solution, pouring this solution in a poor solvent for the polymer, recovering the resulting precipitate, and subjecting it to thermal cyclization reaction to provide the objective polyimide.
However, when polyimide microfine powder is to be produced by this technology, the polymer block must be recovered after the imidation reaction and mechanically pulverized, thus introducing a complicating factor into the production process. Moreover, mechanical pulverization yields only more or less coarse particles and can hardly provide a monodispersed system of discrete particles. In addition, the above technology does not lend itself well to the control of particle morphology and size distribution.
Thus, there has been a standing demand for development of a technology for producing polyimide microfine powder capable of providing a monodispersed system.